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. 2019 May 2;9:6860. doi: 10.1038/s41598-019-43284-6

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© The Author(s) 2019

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.

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Transmission electron microscopy image of type III acinus from unfed Ixodes ricinus female salivary glands highlighting the cellular composition and associated axons. (A) Cross-section through the middle of the acinus reveals common axon positions (red), basal epithelial cells (ECs, dark blue), myoepithelial cell (MC, pink), ablumenal epithelial cells (AECs, aqua-blue) and lumen (yellow). Different granular cells at both basal and apical regions of the acinus are left in black and white. Insets in A are magnified in (B,H,I). (B) The acinar valve region. Apparent microtubules (black arrows) in the MC and (white empty arrows) in ECs are shown. EC labyrinthine junctions (black arrowheads) are closely associated with the acinar duct (AD). Basal canaliculi (white arrowheads) of ECs are evident on the AD apex. The blue asterisk shows the nucleus of the EC. The basal axon is indicated by the red asterisk. (C–E) The long AEC (aqua-blue asterisk) projections (aqua-blue arrows) between two granular cells (yellow squares) reaching the surrounding area of the AD. Inset in (C) is magnified in (D,E). (D–E) A prominent axon (red asterisk) containing neurosecretory vesicles is enclosed by ECs (blue arrows), AEC projections (aqua-blue arrows), and a granular cell (yellow arrows in E). Note that axons are never in direct contact with the AD. (F) Septate junctions (black arrowheads) present at the site of interdigitation between neighboring ECs. (G) The acinar valve (V) is covered by two layers of cells, the ECs (blue arrows) and the MC. Labyrinthine junctions (black arrowheads) of the ECs closely surround the AD. (H–J) The axons (red asterisks) are in direct contact with the MC’s finger-like projections which radiate between granular cells (yellow squares) and fine projections of AECs (aqua-blue arrows in H,I). Aqua-blue asterisk in H shows the nucleus of AEC. The MC nucleus (black asterisk) is shown in (J). N: nucleus; L: lumen. Specimens were prepared by HPF/FS and a modified OTO method with en bloc hafnium chloride (A,B,H–J) or uranyl acetate (C–G) staining. Scale bars: 5 μm (A), 2 μm (B–D,G–J), 500 nm (E).